Capacitive sensing of fingerprints provides for collection of fingerprint information in two dimensions, such as an image of a fingerprint, or portion thereof. More specifically, capacitive sensing allows a fingerprint recognition device to determine the ridges and valleys of the user's finger, in response to relative capacitances measured between the user's finger (such as on the epidermis of the user's finger) and a capacitive plate in the fingerprint recognition device.
A first problem in the known art is that, should the user's finger be positioned incorrectly, or even merely non-optimally, the fingerprint image collected by the fingerprint recognition device may fail to be fully adequate. For a first example, when the user's finger is too far from the fingerprint recognition device, or when the user's finger exerts too much pressure on the fingerprint recognition device, the fingerprint image may have inadequate resolution or be too blurry. For a second example, the user's finger may be positioned with only part of the finger in proper position to be sensed, with the effect that only a partial fingerprint image is collected by the fingerprint recognition device. For a third example, the user's finger may be oriented improperly or at an unexpected or unusual direction with respect to the fingerprint recognition device, with the effect that the fingerprint image, while otherwise reasonably crisp and complete, is not easily identified.
A second problem in the known art is that the fingerprint image collected by the fingerprint recognition device may be subject to noise. Noise can result from multiple sources. For a first example, noise can result from ambient electromagnetic noise near the fingerprint recognition device. For a second example, noise can result from electromagnetic noise within the circuitry of the fingerprint recognition device. For a third example, noise can result from dust or other particulate matter interfering with operation of the fingerprint recognition device. Noise can interfere with the quality of the fingerprint image, such as by altering the ridges and valleys identified by the fingerprint image.
Each of these examples, as well as other possible problems, can cause difficulty for the fingerprint recognition device. For a first example, the fingerprint recognition device may have difficulty collecting adequate fingerprint information from the user, with the effect that enrollment of the user's fingerprint image for later recognition can be impaired, or if the user's fingerprint has already been enrolled by the fingerprint recognition device, with the effect that recognition of the user's fingerprint image can be impaired. For a second example, the fingerprint recognition device may have difficulty correlating multiple fingerprints from the user, with the effect that determining a reliable fingerprint for later recognition can be impaired.